Conventionally, as a submerged membrane separator 1 of this type, there has been a submerged membrane separator that is submerged in a treatment tank and separates activated sludge and treated water as shown in FIG. 21. The submerged membrane separator 1 includes a box-type casing 2 opened at the top and the bottom, a plurality of membrane cartridges 3 arrayed in parallel to one another in the casing 2, and an air diffuser 4 provided below the membrane cartridges 3.
The membrane cartridges 3 have filtration plates 12 and filtration membranes 13 attached to both sides of the filtration membranes 12. The air diffuser 4 is configured to diffuse air from a diffusion pipe. A pressing plate 5 for preventing lifting of the membrane cartridges 3 is attached to the upper part of the casing 2.
Consequently, during a filtration operation, the inner sides of the membrane cartridges 3 are depressurized while air is diffused from the air diffuser 4. Therefore, sludge or the like in a liquid to be treated is caught by the filtration membranes 13 of the membrane cartridges 3. Permeate permeating through the filtration membranes 13 and flowing into the inner sides of the cartridges 3 is collected as treated water 19 in a water collecting pipe 16 from water intake nozzles 14 through connection pipes 15.
In this case, upflow 17 is caused among the membrane cartridges 3 by bubbles of the air diffused from the air diffuser 4. Matters adhering to the membrane surfaces of the membrane cartridges 3 are removed by this upflow 17. At this point, an upward force acts on the membrane cartridges 3 due to the upflow 17. However, the membrane cartridges 3 can be prevented from being lifted owing to the pressing plate 5.
However, since there is a space between the upper end of the membrane cartridge 3 and the pressing plate 5, unfortunately, the upper end of the membrane cartridge 3 is brought into contact with the pressing plate 5 by the upflow 17, the membrane cartridge 3 minutely vibrates, and the upper end of the membrane cartridge 3 repeatedly comes into contact with the pressing plate 5 and is worn.
As a measure against the problem, as shown in FIG. 22, a pressing plate 6 is formed by a tabular supporting portion 7 and a comb-like pressing portion 8 of an elastic material bonded to the tabular supporting portion 7. The tabular supporting portion 7 is fixed to the upper end surface of the casing 2 by a bolt 9 and a nut 10. The comb-like pressing portion 8 has a plurality of protrusions 8a. The protrusions 8a are pressed into spaces between the membrane cartridges 3.
Consequently, the membrane cartridges 3 are pressed from above by the pressing plate 6 and are immovably secured. Therefore, it is possible to prevent vibrations of the membrane cartridges 3.
The submerged membrane separator including the pressing plate as explained is described in, for example, Japanese Patent Application Laid-Open No. H10-137556.